In recent years, automotive manufacturers have begun to increase the utilization of air bag safety devices in their vehicles. Therefore, there has been a continuing effort to improve the safety, performance and durability of these life-saving devices.
An automotive air bag is comprised of an inflatable/deflatable bag, an inflation device, and an impact sensor. Inflation is most often provided by a sodium azide propellant positioned within the inflation device which is ignited in response to activation by an impact sensor. The combustion of this propellant yields nitrogen gas to provide rapid (about 0.2-0.5 sec) inflation of an air bag. Inflation is followed immediately with a deflation cycle. Without such an immediate deflation, the impact of a passenger upon the air bag itself could cause substantial injury.
One means of providing a deflation function for an air bag is to provide for partial air bag gas porosity. Currently, this approach is provided by utilizing a porous air bag made of uncoated fabric or non-porous fabric with vents. When utilizing such fabrics, it is required that the exhaust rate of the air bag fabric be less than the inflation rate after propellant ignition, otherwise a positive pressure would not occur within the bag. The entire inflation/deflation cycle spans approximately 0.6 seconds.
There is concern regarding several problems associated with the partial air bag porosity approach as it applies to current art on passenger and/or driver side air bags. The primary concerns centers upon the possibility that fabric yarn distortion or separation may compromise the inflation cycle. These conditions can result in an unequal volume fill of the bag, higher than acceptable gas leak rate at areas of distortion or separation, or jet stream effects that result in improper bag registration at the peak of the inflation event.
Fabric coatings have been used to provide a certain degree of yarn stability to air bag material but, at the same time, such coatings severely decrease bag porosity and increase cost. It would be highly advantageous to have an air bag material that would maintain yarn integrity during air bag inflation while providing the required air bag porosity.
The inflation/deployment of a driver side air bag: for example is both rapid (about 0.2 seconds) and aggressive, reaching a static pressure of 3 to 5 psi. The impact of the driver upon an air bag increases internal air bag pressure to from about 9 to 12 psi. It is extremely important during the deployment cycle that sewn seams utilized to fabricate the bag are not compromised. It has been demonstrated that a weakness of sewn seams due to defective workmanship, fabric construction, or treatments/coatings, to fabric substrates or untreated fabric substrates could result in a condition referred to as "combing" where separation between the fibers occurs adjacent to the seam.
Stress separation during inflation and impact events resulting in separation or yarn pull-out could compromise the air bag performance or lead to catastrophic failure of the bag and/or secondary injury to the driver or passenger.
It would therefore be highly advantageous to provide an air bag fabric substrate capable of maintaining the integrity of sewn seams during packaging, compaction (for module positioning), and inflation. Preferably, such a substrate would also possess sewn seams which are capable of withstanding yarn pull-out or other catastrophic failure throughout its service life.
It is essential to provide high pliability in an air bag fabric to facilitate packageability. Furthermore, inflation time during an inflation event depends, in part, upon air bag pliability. Less pliable fabrics increase resistance to an air bag deployment resulting in a longer inflation event. Inflation times are specific to a given vehicle specification in order to provide maximum protection for the driver or passenger. It would be advantageous for a coated air bag fabric to exhibit substantially the same pliability as an uncoated air bag fabric.
Air bags are often fabricated from multiple patterns of coated or un-coated material which are sewn, bonded, or sewn and bonded. Once inflated, this manufacturing process provides a pre-selected air bag shape in accordance with interior cabin architecture and the position of the driver or passenger.
When air bag patterns are made from loom state or finished fabrics such as nylon or polyester, difficulties are encountered. These materials have a tendency to distort or fray during conventional pattern cutting operations. To avoid these problems, laser technology or hot melt dies have been employed. Since economy and high through-put are of high priority in enabling universal availability of air bags in automobiles, it would be highly desirable to provide a method for treating fabrics (such as nylon and polyester), so as to minimize fraying and distortion (filament separation in the yarn or yarn separation from the fabric). Such a method would allow economical, conventional pattern cutting technology to be utilized and would thereby help reduce the cost of air bag manufacture.
As discussed above, the inflation of an air bag is most commonly provided by the ignition of a sodium azide propellant. The ignition of this propellant results in a highly exothermic reaction. Design considerations demand that the temperature of nitrogen gas formed by this reaction be rapidly cooled in order to prevent damage to the air bag or facial injuries to the occupant. Current air bag inflators are efficient at reducing gas temperature; however, there are still risks associated with pyrotechnic damage to the air bag. Bag damage, secondary injury, or catastrophic failure of the air bag are all potential risks which must be addressed.
1. Particulate matter (cinders)--Inflator designs include primary and secondary filter medias for the purpose of capturing solid matter resulting from the rapid combustion of the solid fuel. The efficiency of the filters are quite high; however, the potential exists for hot cinders to be propelled into the environment of the bag resulting in microscopic "pitting" of the fabric substrate. This occurrence, in itself, may not necessarily result in a decrease of the bag's performance. However, there is concern that the temperature of the cinders could ignite the fabric substrate if it does not have a measurable degree of flame retardancy. PA1 2. Bag/manifold junctions--Air bag assemblies are normally attached to an inflator housing by means of several coated gaskets and metal "O" rings. The gaskets serve as insulators between the housing and air bag material. Since the ignition of an inflator's solid fuel results in exothermic reaction temperatures of from about 1200.degree. to 1400.degree. F., the bag/manifold junction posses a risk of transferring excessive, potentially ignition-producing heat to the air bag material. Therefore, it is of the greatest importance that the air bag fabric substrate exhibit flame retardancy. PA1 3. Deflation drape--After an air bag deployment event, the bag will lose gas pressure and deflate and assume a limp vertical hang position from the inflator housing. In this position, it is possible for the air bag fabric to come in direct contact with hot metal parts of the inflator housing. As described above, failure of the fabric to exhibit a measurable degree of flame retardancy could result in ignition of the air bag substrate. PA1 1. An active hydrogen alone, resulting in the formation of a cation position; PA1 2. An active hydrogen with one electron resulting in a substrate free radical position; or PA1 3. An active hydrogen and both electrons resulting in the formation of an anion position on the substrate. PA1 1. Radical formation--The active hydrogen may be abstracted by a graft initiator to form a free radical carbonyl group located in the polyester chain. PA1 2. Initiation/extension--The free radical carbonyl group thereafter reacts with either a first component or a second component (e.g. CH.sub.2 .dbd.CH--X), so as to graft the component as a free radical upon the polyester chain. PA1 3. Propagation--the grafted free radical component may now, covalently bond to additional components of the same or different species thereby activating additional components to a free radical state, or PA1 4. Termination--may react with another free radical to terminate the polymerization process. Peroxide, for example may be converted by graft initiator to a free radical state for bonding with activated monomer. ##STR2## The process may be terminated by radical combination: ##STR3##
In view of these potential fire hazards, it would be highly desirable for the air bag to possess increased flame resistance without compromising other required air bag performance features such as permeability and pliability.
An automotive air bag system must be designed so as to endure a wide range of environmental conditions without compromise of function or performance. Air bag designs or methods must provide an air bag which is functional at environmental extremes as defined by current automotive standards for performance. Air bag fabrics must also not support microbial growth, demonstrate appreciable changes in base physical properties, show appreciable change in pliability, or compromise static and dynamic requirements of the air bag when conditioned over any given environmental event.
What is needed is a vehicular air bag comprised of a fabric exhibiting increased structural integrity effectively reducing yarn distortion, pull-out and related stress failures while also providing increased resistance to seam failure and combing. Furthermore, it would be highly advantageous to provide an air bag fabric which could be effectively die-cut without filament or yarn pull-out or fraying, or fabric distortion. It would be further advantageous to provide improved flame retardance in an air bag.